The present disclosure relates generally to systems and methods for magnetic resonance imaging and, in particular, to systems and methods for functional brain mapping.
Neurosurgical procedures for patients with brain afflictions often require a balance between minimizing post-operative functional deficits and maximizing the benefits of the intervention. Each year, about 200,000 people are diagnosed with primary or metastatic brain cancer, with an approximate 0.6% lifetime risk, where the best hope for survival includes maximizing tumor resection while preserving as much normal functioning brain as possible. Epilepsy is another common disease affecting about 0.7% of the population, with about a third of the cases amenable to surgical treatment. To avoid post-surgical morbidity, routine surgical planning is typically associated with a process for obtaining the functional organization of brain. This involves several major difficulties, including determining the dominant hemisphere for the identification of important functions, mapping healthy eloquent cortices, and localizing epileptic foci or defining borders for brain lesions.
Information about the anatomical relationship between eloquent cortex and an area to be excised is extremely valuable in planning the operation. For example, in patients with frontal or parietal lobe lesions, it is often important to map the proximity or involvement of the primary motor and sensory cortices. Also, in frontal and temporal epilepsy patients, mapping language and memory systems is usually required. To map the eloquent cortices, invasive cortical stimulation is often managed peri-operatively in the awake patient or in the pre-surgical patient with subdural grids implanted. Under these conditions, stimulation-induced disruption provides information about the location of eloquent cortex. With subdural grids, it is also possible to conduct evoked potential studies to identify the functional networks. Although cortical stimulation can accurately map many functional systems, it is far from perfect. Besides the common drawbacks of all invasive techniques, cortical mapping usually occurs a short time before the planned resection, leaving little time to analyze the results and discuss options. It also lacks of information about deep brain structures because the subdural grids only record the electrical potential on the brain surface. When language and memory functions are considered, patient participation is required.
More recently, fMRI has been offered as a non-invasive means of mapping eloquent cortices. The basic approach involves an imaging session while a patient performs a task set designed to target a single domain such as language, memory, or motor function. The obtained images are then used prior to the surgery to identify regions of functional activity. The approach is powerful because it allows detailed assessment of functional anatomy in a timely manner that includes deep brain structures. However, there are significant limitations. First, some patients have difficulty performing the required tasks, especially those who have developmental brain disorders, altered levels of consciousness or other functional impairments. If a patient is not able to perform the prescribed task, then functional mapping may be unreliable or prove impossible. Second, specific task sets must be performed to target distinct functions (e.g., language versus motor function). While optimization is possible to allow efficient cycling through multiple functional domains, it is presently not possible to simultaneously map multiple brain functions. Even within the motor system, task epochs must alternate between separate motor acts (e.g., hand versus tongue movements) to map their distinct anatomic locations.
Therefore, given the above limitations, there is a need for systems and methods for robust determination of a functional organization for an individual brain.